1. Field of the Invention
The present invention relates to a film capacitor. More particularly, the present invention relates to an improvement of a stacked film capacitor including a pair of external electrodes that is prepared by a spraying process on a pair of side surfaces of a stacked capacitor element and a resin protective film on a side surface of the remaining side surfaces, and an advantageous method of producing the stacked film capacitor.
2. Description Of Related Art
Conventionally, as one type of film capacitor for use in an electric device, there is known a stacked film capacitor. The stacked film capacitor includes a capacitor element that has a square shape (cuboid or cubic shape) which allows the stacked film capacitor to be surface mounted. Thus, the stacked film capacitor is expected to be increasingly demanded because it can satisfy the demands for smaller film capacitors and high density mounting thereof, which results from recent remarkable downsizing of electric devices.
The stacked film capacitor is generally produced by the following method. Specifically, first, a long capacitor base is formed which includes a plurality of dielectric layers each consisting of a insulating resin film which is formed of polypropylene or polyethylene terephthalate, and a plurality of vapor-deposited metal film layers as internal electrode films, which are stacked with each other so as to be arranged alternately. Second, a pair of external electrodes is formed by a spraying process on side surfaces opposing to each other in a direction perpendicular to the longitudinal direction of the long capacitor base. Subsequently, the long capacitor base is cut into some sections in the longitudinal direction to obtain a plurality of capacitor elements. Then, a terminal or the like is attached to each of the external electrodes of the plurality of capacitor elements and each of the plurality of capacitor elements is contained and sealed in a predetermined case. Thus, a plurality of intended film capacitors are obtained.
Since the plurality of capacitor elements are cut from the capacitor base at one time and used to produce the film capacitor, the film capacitors obtained as above can be efficiently produced. However, if cut surfaces of the capacitor element cut from the capacitor base are left exposed, foreign substance may be attached to the cut surfaces before the capacitor element is contained in the case. It may cause a conduction defect.
In view of the above, as disclosed in JP-A-2001-297945, for example, resin protective films are formed on the cut surfaces of the capacitor element cut out from the capacitor base, i.e., opposing side surfaces of the capacitor element on which no external electrode is formed. By the formation of the resin protective films, it can be prevented that the foreign substance is attached to the side surfaces other than the surfaces on which the external electrodes are formed. Further, it also can be effectively prevented that the conduction defect is caused by the foreign substance. In the film capacitor disclosed in JP-A-2001-297945, the resin protective films are formed on the capacitor element by vacuum deposition. Thus, unlike the case in which the resin protective film is formed by dipping or the like in air, even during the deposition operation of the resin protective film, it can be effectively prevented that foreign substance is attached to the side surfaces of the capacitor element other than the surfaces on which the external electrodes are formed.
However, the conventional film capacitor inherently includes the following problems. Generally, as is well known, since it is difficult to evaporate polymer material, it was not easy to form a polymer thin film by vacuum deposition. Thus, in the conventional film capacitor in which the resin protective film is formed by deposition, the resin protective film is formed of a thin film having a relatively low molecular weight. Consequently, it was difficult to secure the sufficient durability of the resin protective film.